Gábor Varga

Self-consistent real three-dimensional simulation of vertical-cavity surface-emitting lasers

A self-consistently coupled three-dimensional dynamical model is presented for the first time, to our knowledge, to simulate noncircular vertical-cavity surface-emitting lasers. The electric, thermal, and optical processes are formulated using finite-volume-method discretization adopted on a unified mesh consisting of prism elements. Steady-state and dynamical calculations are shown for specially designed structures. An efficient numerical treatment enables one to perform large-scale calculations on single computers.

Indoor location services and context-sensitive applications in wireless networks

The present research was conducted at the Budapest University of Technology in the field of indoor location using radio waves of Wi-Fi networks with a focus on practical application issues. Our goal was to enhance and combine existing algorithms and create an implementation that is efficient enough to enable real-time operation in 3D space in multi-level office environments while retaining the accuracy of more complex systems and allowing the addition of valuable context-sensitive features.

Real three-dimensional optical simulation of non-circular VCSEL structures with finite volume method

This work presents a real three-dimensional (3D) solution of the scalar Helmholtz-equation for VCSELs with finite volume method (FVM). The advantage of this formulation is that the order and structure of the system matrix can be reduced compared to vectorial finite element approach, and thus the feasible problem size can be significantly increased. Moreover, the combination with three-dimensional current flow and thermal analysis can be realized with the same discretization without extra numerical efforts. Results show the calculated fundamental mode profile on a sample VCSEL with square oxide aperture.

Optical simulation of vertical-cavity surface-emitting lasers with noncylindrical oxide confinement

Vertical-cavity surface-emitting lasers (VCSELs) have a high potential in telecommunication applications. We have developed two extensions of the weighted index method (WIM): the empirical effective-radius method (ER-WIM) and the hybrid analytical-numerical method. The ER-WIM defines two effective radii for a noncircular aperture. The two models proposed here aim to simplify the problem to a physically acceptable level. They are still reliable but computationally more feasible because they are significantly faster than a full-3D FEM and never require large computer resources.

Real three-dimensional dynamical VCSEL simulation with spatially distributed noise sources

A self-consistently coupled three-dimensional dynamical model is presented for non-circular vertical-cavity surface-emitting lasers. The electric, thermal and optical processes are simulated using the finite volume method formulation adopted on a unified mesh consisting prism elements. Spatially distributed noise sources are incorporated into the quantum-well dynamics providing relative intensity noise spectrum.

Computer simulation of the operating pressure of tungsten halogen lamps

The small bulb of the tungsten halogen lamp often operates at several hundred degrees Celsius and an internal pressure higher than atmospheric pressure. The cold lamp contains a gas filling at about 3 bar; the pressure during operation is much higher than that. The objective of our project is to analyse the internal operating conditions (such as the pressure and the thermal behaviour) of certain halogen lamps. The actual measurement of the operating conditions is far from easy. Rather, a computer model has been developed to evaluate the operating conditions in these lamps, based on the substantial partial differential equations of the free convection problem. This model can help determine the temperature distribution, the velocity field and the pressure in these lamps.The free convection has been simulated by the following two appropriate equations: the convection–conduction equation—it is the heat transfer equation for convecting materials—and the incompressible Navier–Stokes equation. We could get the temperature field from our results, which helped us to calculate the operating pressure of the lamps. The pressure dependence on dimensionless characteristic numbers was evaluated.

Simple hard corrugated wall model computations

Abstract The simplest way to take into account the periodic characteristics of the surface by the hard corrugated wall (HCW) model is investigated. This model gives basic information for the calculation of elastic scattering applicable to more difficult potentials. In applying the HCW model a basic requirement is that the mathematical method should be simple enough and not demanding too much computing time. A modified version of the GR method is presented that can provide the appropriate exactness by relatively little computing work. Our procedure is shown to be convergent, it is compared with other methods. The use of a special sequence of points allows us to solve only a rather small number of equations. For this method the computing time depends strongly on the number of points, which itself depends on the number and on the position of selected channels.

Real three-dimensional spatio-temporal VCSEL simulation

We present a real three-dimensional (3D) dynamically combined model for the first time, realized with finite volume method on a flexible, triangle-based mesh. The simulation consists of a current flow, a heat flow and optical mode solvers, coupled to each other self-consistently in the quantum well laser dynamics. The transport of free carriers across the mirrors satisfies the Laplace-equation, where heterojunctions are modeled with increased resistivity in the axial direction.

Computer simulated thermal energy atomic scattering on solid surfaces

Abstract Thermal energy atomic scattering on solid surface is investigated by computer simulation. The atomic beam is described by Gaussian wave-packet as an ensemble of independent particles. The atom–solid surface interaction is characterised by an appropriate interaction potential. The interaction potential provides the properties of ideally periodic and disordered surfaces, respectively. The scattering process is governed by time dependent Schrodinger equation that is solved numerically in the case of two-dimensional (2D) and (3D) co-ordinate space. The above-described model provides not only the quantitative intensity distribution but also the animation of the interaction as the time passes. The effect of relative velocity spread, surface disorder and thermal vibration have been investigated efficiently.

Resolution and transfer width of thermal energy atomic scattering from solid surfaces (TEAS)

Abstract The resolution of TEAS has been investigated as a function of energy spread of atomic beam. The model calculations have been executed within the framework of time dependent Schrodinger equation. The energy spread of realistic atomic beam has been taken into account by a wave-packet. The wave-packet describes the atomic beam as an ensemble of independent particles by quantum mechanics. Taking ideally periodic surface the resolution of diffraction peaks increases when the energy spread is decreased. This fact underlines the higher efficiency of the supersonic atomic source than the effusive atomic source. Furthermore the transfer width of experimental equipment increases—when the atomic beam monochromaticity is also increased—according to the concept of the transfer width. The relation between the transfer width and the size of the period of the surface topography significantly determines the resolution of the diffraction pattern.